mm: declare some external symbols

Declare 'bdi_pending_list' and 'tag_pages_for_writeback()' to remove
following sparse warnings:

 mm/backing-dev.c:46:1: warning: symbol 'bdi_pending_list' was not declared. Should it be static?
 mm/page-writeback.c:825:6: warning: symbol 'tag_pages_for_writeback' was not declared. Should it be static?

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index f1b402a..4ce34fa 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -111,6 +111,7 @@
 
 extern spinlock_t bdi_lock;
 extern struct list_head bdi_list;
+extern struct list_head bdi_pending_list;
 
 static inline int wb_has_dirty_io(struct bdi_writeback *wb)
 {